This invention relates generally to electromagnetic clutches, such as for use in controlling the transmission of power from an automobile engine to a refrigerant compressor in an automobile air conditioning refrigerating system, and more particularly, to an improved connecting structure between an armature plate and a hub mounted on a compressor shaft for transferring the rotation of a pulley to the compressor shaft.
The general structure of an electromagnetic clutch for use between an automobile engine and refrigerant compressor is disclosed in U.S. Pat. Nos. 3,044,594 and 3,082,933. The electromagnetic clutch disclosed in these patents, which is illustrated in FIGS. 1-2 of the annexed drawings, comprises pulley 1 rotatably supported on a tubular extension of a compressor housing through bearing 2. The pulley is rotated by a belt coupled to the automobile engine. Pulley 1 is made from a magnetic material in which one or more concentric arcuate or annular slits 3 are formed to define a plurality of concentric annular magnetic poles. Drive shaft 4 extends through a tubular extension in the compressor housing and hub 5 is fixed to the terminal end of the drive shaft. Annular armature plate 6, which is concentric with hub 5 at an annular radial gap, is joined to hub 5 by a plurality of leaf springs 7 so that armature plate 6 faces the annular concentric poles of pulley 1 at a predetermined axial air gap. One end of each leaf spring 7 is secured on the axial end surface of hub 5, together with stopper plate 9, by rivet 10 and the other end is secured on the axial end surface of armature plate 6 by rivet 11. Magnetic coil 8 is mounted on the tubular extension of the housing to generate magnetic flux to attract armature plate 6 to the magnetic poles. Thus, when magnetic coil 8 is energized and the engine is driving pulley 1, drive shaft 4 rotates together with pulley 1. However, when magnetic coil 8 is not energized, even though pulley 1 is rotated by the engine, drive shaft 4 of the compressor is not driven.
In the above prior art construction, armature plate 6 is joined to hub 5 by a plurality of leaf springs 7 and hub 5 is fixed to drive shaft 4. At the moment when armature plate 6 first begins to rotate in response to the magnetic forces generated by pulley 1 and magnetic coil 8, an impact force occurs. This impact force is the result of the sudden initiation of rotational movement of the armature plate because of its magnetic coupling to the rotating pulley. However, since this impact force is not cushioned by the rigid connection of the leaf springs between the armature plate and the drive shaft, a sudden torque is transmitted to the drive shaft of the compressor.
In order to avoid the application of a sudden torque to the drive shaft, it also is conventional to form each of the leaf springs in the shape of an arc to absorb the impact force by bending the arc-shaped leaf springs 7', as shown in FIG. 3. However, since the arc-shaped leaf springs 7' have limited elasticity, the endurance or useful life of the leaf springs is inferior to conventional straight leaf springs. Furthermore, the arc-shaped leaf springs are disposed at an angular offset to be drawn or pulled generally along their lengthwise dimension in the direction of the rotation (in the figures, the direction of rotation is shown by arrows). If the clutch is rotated in a direction opposite to the normal direction of rotation, shearing stress occurs in the leaf spring. As a result, the useful life of the clutch is reduced.